Telephone networks comprise a series of interconnected subsystems that are linked together at points called interfaces. These interfaces provide a link between old and new equipment and allow for simplified design and maintenance. A local loop is an example of an interface that connects a subscriber's telephone set and the central office.
The U.S. Federal Communications Commission (FCC) and counterpart regulatory agencies in other countries require, among other things, electrical isolation between the line side and the user devices on the user side. Electrical isolation protects the line side from damage transmitted from the device side and vice versa.
The isolation between the line side and the device side is often accomplished within the interface circuit. A modem is an example of an interface circuit that may include circuitry that provides electrical isolation from the line in addition to the signal modulation and demodulation function of the modem. Isolation transformers, optical coupling, and capacitive coupling are all examples of known methods of isolating the line side from the device side.
It is known to include in the interface circuit a discrimination circuit to identify the occurrence of signals is indicative of specific “events” that occur on the telephone line and discriminate between the different signals to determine what the particular event signifies.
FIG. 1 is a block diagram of a typical telecommunication system 5 showing the connection between a subscriber and a central office that controls the telecommunication system. Central-office equipment 10 on the line side 15 of the telecommunication system 5 is connected to user device 20 (e.g., telephones or computer terminals) on the device side 25 of the telephone system 5 via an interface circuit 30.
Many components (e.g., data access arrangements (DAAs) or CODEC's) of telephone interface circuits are PSTN line-powered circuits that also require isolation from low-voltage power supplies. Because of this required isolation, line powered interface circuits will not function until they are connected to a power source, i.e., the interface circuit must be activated when it is needed to connect the line side to the device side. When a circuit on the line side is placed in the “on-hook” state (e.g., a telephone receiver is placed in its cradle) the local loop is opened and almost all of the power to the interface circuit is cut off. Activating the DAA or CODEC requires that some power must be drawn from the loop or from another source. While in the on-hook state a small amount of current (idle-state loop current) can be drawn for a short period of time from the TIP/RING line to register that an event has occurred on the PSTN TIP and RING line. However, it is extremely inefficient to draw the extra power unless it is really needed, i.e., it is extremely wasteful of time and resources to power up the DAA or CODEC when a transient noise signal occurs on the TIP and RING line.
The primary events that the interface must detect while in the on-hook state are the application of a TIP and RING signal and a polarity reversal of TIP and RING DC voltage, either of which may be used to signal the need for more power. A problem can arise because ordinary transients on the line can have electrical characteristics very similar to those of actual events. To discriminate between transients and actual events, prior art interface circuits employ the previously mentioned discrimination circuitry that determines the exact nature of every signal introduced thereto, but these additional circuits draw higher amounts of power. While drawing additional power is acceptable on a temporary basis, the amount of power available is limited. Discrimination circuits must operate at power levels no higher than the amounts determined by regulatory agencies as “satisfactory” on-hook leakage currents. Different amounts of leakage current are allowed for short periods of time. As an example, ringer AC current may be rectified and used as a source of power. Regulatory agencies allow higher leakage currents during Calling Number Delivery (Caller-ID).
In addition to determining what an event is, the event signals themselves must be transmitted from the device side to the line side. A problem arises because high voltage isolation prevents the transmission of the event signals from the device side to the line side. Prior art circuits avoid this problem by using optical couplers to transmit the signal caused by the actual event to the low voltage interface. These circuits employ a general purpose optical coupler with an LED input and a photo-transistor output. These optical couplers require “light pipes” which are cavities on the chip between the emitter and the detector of the optical coupler to allow the light to pass between the two. These light pipes increase the size and cost of the interface circuit. Capacitive coupling, another known isolation method, allows the circuit size to remain small and low cost, but the rate at which the events occur are too slow to accurately transmit them from the device side to the line side using capacitive coupling.
To avoid powering up the discrimination circuitry except when it is needed to process an actual event, it would be desirable to employ a preliminary circuit that, before invoking the discrimination circuitry, distinguishes between actual events (e.g., a ring signal, a polarity reversal, an audio signal) and noise (transient spikes on the line caused by a variety of sources, e.g., lightning, battery noise, etc.). Accordingly, there remains a need for a simplified, smaller, and less costly detection circuit that can operate on the minimal amount of power available when the circuit is in the on-hook state to preliminarily distinguish between actual and noise events, and transmit event signals between the device side and line side while maintaining electrical isolation between the device side and the line side.